There were three medal slots added -- 308, 309, 310. Timeless River Mickey is a 5* that takes up 309 and its evolved form would take up 310, leaving 308 a mystery. It could hypothetically be a 4* Timeless River Mickey, which might mean there are more chances to get it down the line, but that seems unlikely.
Other than that we don't really know what it will be. It being just one spot suggests, to me, that it will be a Support Medal.
The Skills to Thrill Medal Draw ends tonight. We'll be seeing a new Medal Draw added, which might include this mystery medal, but we won't know until then I imagine.